Horticulture Clinic

Several types of leaf spots affect aspen, cottonwood and poplars. Caused by fungi, these diseases develop readily in cool, wet weather. Infection occurs in spring, but symptoms usually don’t appear until mid to late summer. Heavy infested trees may drop leaves prematurely. Overall health of the tree can be affected during severe outbreaks.

Disease symptoms vary. Marssonina leaf spots are dark brown dots with yellow halos. In prolonged wet weather, spots may fuse to form larger black patches. Septoria spots are tan and circular with dark margins. Cibornia first appears as tan to brown areas on leaf surfaces. As the season progresses, the spots take on a black, oval, raised appearance, giving the disease its common name, ink spot. Melampospora rust appears as small, orange-colored pustules.

One of the best ways to manage these diseases is now, in the fall. Rake up fallen leaves and dispose of them. The diseases over-winter in the fallen leaves. In the spring, when temperature and moisture are appropriate, fungal spores are released from the infected leaves. Spores are then blown or carried by splashing water to susceptible tissue, causing new infections. Raking leaves removes inoculum from the site.

Some other tips:

• When planting, space plants according to recommendations for the species. Trees planted too closely increase the humidity within the canopy, creating conditions ideal for infection.
• Keep sprinkler water out of leaves, so they stay dry. Splashing water can also spread spores between leaves, causing secondary infections.
• Prune to thin trees, increasing air circulation in the canopy, keeping leaf surfaces dry.